Such systems are principally known in the art. It is especially known that the gain μ of a scintillation detector system, comprising a scintillator, a photocathode and a photomultiplier tube (PMT) together with an evaluation system is subject to a change in gain over time. The gain change of the overall system is substantially effected by the gain change of the PMT. That gain change is due to environmental changes, i.e. a modification in temperature over time or other environmental factors.
In order to stabilize the gain of the PMT, it is known in the art to conduct several measurements over time and to compare the results. An initial or reference measurement may take place at beginning of the first measurement of nuclear radiation, for example using a calibration source with well-known energies of the emitted gamma radiation. The light signals, produced by the gamma radiation in the scintillator crystal, are proportional to the energy deposed in that crystal. The light signals do then hit the photocathode, that photocathode emitting electrons, which are collected by a PMT. A PMT consists of a series of dynodes and a final anode. The—usually very few—photoelectrons from the photocathode are accelerated towards the first dynode where they produce a multitude of electrons, being emitted from that first dynode. Those electrons are then accelerated to the next dynode, where their number is again multiplied by the same factor, those electrons being led to the next dynode and so on, until they finally reach the anode of the PMT, where a current signal is measured, being proportional to the charge of the multitude of electrons. That charge is proportional to the amount of light, generated in the scintillator and therefore proportional to the energy deposed by the gamma radiation in the scintillator.
The resulting charge signal is then further processed and usually stored in a multichannel analyzer (MCA), each channel of that MCA corresponding to a specific radiation energy, deposed in the scintillator crystal. An accumulation of such energy signals results in an energy spectrum, each line in that spectrum corresponding to a specific energy deposed in the detector system.
For most applications it is of interest to obtain the best resolution in energy a system allows. One of the problems, leading to a decrease in energy resolution is the gain shift, which is to be avoided therefore.
In order to do so, it is known to measure the gain at different times, using gamma radiation with known energy. This gamma radiation with known energy may be emitted by a calibration source, or may be another known energy, being present in the spectrum to be measured anyway. The gain of the two measurements at different times is compared and the signals are corrected by the difference, therefore multiplying all signals by a so-called gain correction factor, thereby stabilizing the overall system.
DE 2 826 484 does propose to use a quantum reference by utilizing the single photo electron charge. Single photo electrons are emitted as thermionic current from the cathode of a photomultiplier tube. Those single electron induced pulses are detectable at a pulse height several orders of magnitude of below the equivalent lowest scintillation pulse range for gamma energies. DE 2 826 484 proposes to split the PMT output signal into two channels, one channel with an amplification suitable for the normal gamma energies to be measured, the other channel with a higher amplification in order to make the SEP's visible also. The two channels are distinguished by applying different pulse width discrimination, thereby separating pulses with a broad timing constant of 230 ns and those with a shorter timing constant of about 30 ns, the latter being those of the single electron induced peaks, which is seen by DE 2 826 484 at a 800 eV gamma equivalent. Stabilization was performed by comparing count rates in low energy ROIs without determining the peak distribution and without extracting stabilization parameters.
A specific disadvantage of such a method is that the PMT has to be set to an amplification being high enough to identify the single electron signals. As this does also amplify the gamma induced pulses much more than necessary, such a detector system can be used only for radiation sources with a low count rate. It is also not possible to measure high energy gamma radiation with such a system.
It is also known to use artificial light pulses instead of light pulses, generated by the scintillation crystal following the absorption of radiation energy. Such an artificial light source may be an LED.
All the methods known in the prior art require troubling extra references in the form of complex light sources, thermometers and/or undesired radioactive sources. They also suffer from the fact that they do not cover the complete dynamic count rate regime of a spectrometer. One has to know either a specific—constant—line (energy) in the spectrum to be measured or to use a calibration source, thereby interrupting the measurement from time to time. In addition, especially at high count rates and/or high gamma radiation energy, it may be difficult to obtain a stabilized spectrum at all.
The aim of the present invention is therefore to avoid the above-mentioned disadvantages and to provide a self-stabilizing scintillation detector system without the need of identifying specific lines in the output spectrum, identified as calibration sources, and to correct the gain on the basis of the shift of those lines.